The present invention relates to non-magnetic particles for a non-magnetic undercoating layer of a magnetic recording medium, a non-magnetic undercoating layer containing such non-magnetic particles, and a magnetic recording medium having such an undercoating layer.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been strongly required to have a higher performance, namely, a higher recording density and to reduce the noise level.
Especially, video tapes are required to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the shortwave region have come to be used, and as a result, the magnetization thickness (magnetization depth) from the surface of a magnetic tape has come to be remarkably small (shallow).
With respect to signals having a short wavelength, efforts have also been made to improve the high output characteristics, especially, the S/N ratio, as described in the following literature. For example, Development of magnetic Materials and Technique for High Dispersion of magnetic Powder, published by Kabushiki Kaisha Sogo Gijutsu Center (1982) described on page 74, " . . . in the recording and reproducing characteristics, technical problems in designing a magnetic coating layer so as to produce a high recording density by achieving various requirements in electromagnetic conversion property such as a reduction in the noise level, improvement of the S/N ratio, the sensitivity and the frequency characteristics, and a reduction in the output fluctuation, are (1) to improve the uniform dispersibility of magnetic particles and the magnetic orientation, (2) to increase the packing ratio of magnetic particles in a coating film and (3) to provide a coating film with an excellent surface smoothness and a uniform thickness . . . ", and on page 312, " . . . the conditions for high-density recording in a coating-type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force (Hc) and residual magnetization (Br), . . . and the coating film has a smaller thickness . . . ".
Development of a thinner film for a magnetic recording layer has caused some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to remove the non-uniformity of thickness thereof. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Material for Synthetic Technology-Causes of Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Synthetic Technology)" (1987), published by the publishing department of Technology Information Center, " . . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, . . . since the magnetic layer is formed on the surface of the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved".
Secondly, a problem in the strength of a base film has been caused with a tendency of the reduction in the thickness of a base film in response to the demand for a thinner magnetic layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, " . . . Higher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travelling in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the longitudinal direction and in the transverse direction is now strongly demanded . . . "
Thirdly, there is a problem of too large a light transmittance caused by ultra-fine magnetic particles and a thin magnetic recording layer. Travel of a magnetic recording medium such as a magnetic tape, especially, a video tape is stopped when the video deck detects a portion of the magnetic recording medium at which the light transmittance is large. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultra-fine magnetic particles dispersed in the magnetic coating layer, it is difficult to detect the portion having a large light transmittance by a video deck. As a measure for reducing the light transmittance of the magnetic coating layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic coating layer in the present video tapes.
However, addition of non-magnetic particles such as carbon black not only impairs the enhancement of the recording density but also reduces the magnetization thickness from the surface of the magnetic tape. It is, therefore, unfavorable to add non-magnetic particles to a magnetic coating layer.
To solve this problem, a magnetic recording medium having a base film composed of a non-magnetic substrate and an undercoating layer containing non-magnetic particles has been proposed.
For example, Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 (1988) proposes a magnetic recording medium comprising a non-magnetic substrate, at least one undercoating layer obtained by dispersing non-magnetic particles in a binder, and a magnetic coating layer obtained by dispersing ferromagnetic particles in a binder, wherein the ferromagnetic particles are ferromagnetic iron oxide particles, ferromagnetic cobalt-modified iron oxide particles or ferromagnetic alloy particles, the average major axis diameter of the ferromagnetic particles measured through a transmission electron microscope is less than 0.30 .mu.m and the crystalline size thereof by X-ray diffraction is less than 300 .ANG..
According to the specification thereof, the non-magnetic particles used for the undercoating layer are carbon black, graphite, titanium oxide, barium sulfate, ZnS, MgCO.sub.3, ZnO, CaO, .gamma.-iron oxide, tungsten disulfite, molybdenum disulfite, boron nitride, MgO, SnO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3, a-Al.sub.2 O.sub.3, SiC, cerium oxide, corundum, synthetic diamond, .alpha.-iron oxide, garnet, quartzite, silicon nitride, silicon carbide, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide, tripoli, diatomaceous, dolomite or the like. Among these, inorganic particles such as carbon black, CaCO.sub.3, titanium oxide, barium sulfate, .alpha.-Al.sub.2 O.sub.3, .alpha.-iron oxide, .gamma.-iron oxide and polymer particles such as polyethylene particles are preferable. The particle diameter of such non-magnetic particles is 1 to 1000 m.mu. in the case of granular particles, and 100 m.mu. to 5.mu. in major axis diameter and 1 to 1000 m.mu. in minor axis diameter in the case of acicular particles.
However, as a result of examination of the above-described non-magnetic particles by the present inventors, it has been found that the surface smoothness and the strength are unsatisfactory not only when granular non-magnetic particles are used but also when acicular non-magnetic particles are used.
Japanese Patent Application Laid-Open (KOKAI) No. 4-167225 (1992) proposes a magnetic recording medium produced by forming a magnetic coating layer on the surface of a non-magnetic substrate through an undercoating layer which contains acicular particles having an aspect ratio of more than 3.0 in a resin binder hardened when irradiated with an electromagnetic wave such as radioactive rays and ultraviolet rays.
However, in the case of using acicular .alpha.-FeOOH particles, which are described as the acicular particles in examples thereof, since much crystal water is contained in the surfaces of acicular .alpha.-FeOOH particles, the conformity of the particles with a binder resin and a solvent is so poor that the desired dispersibility is not obtained.
Japanese Patent Application Laid-Open (KOKAI) No. 4-325915 (1992) proposes a magnetic recording medium produced by coating on a non-magnetic substrate with an undercoating layer at least containing non-magnetic particles and a binder, and coating on the undercoating layer in the moistened state with an upper magnetic coating layer containing ferromagnetic particles and a binder, wherein an aspect ratio of the non-magnetic particles is not less than 2.5, the thickness of the upper magnetic coating layer in the dried state is not more than 1 .mu.m and the average particle diameter of the ferromagnetic particles having the major axis diameter is not more than 0.3 .mu.m.
As examples of preferred acicular non-magnetic particles are exemplified non-magnetic metals such as Cu, Cr, Ag, Al, Ti and W and the oxides thereof such as Al.sub.2 O.sub.3 (.alpha.,.gamma.), Cr.sub.2 O.sub.3, .alpha.-ferrite, goethite, SiO.sub.2 (including glass), ZrO.sub.2, CeO.sub.2 and TiO.sub.2 (rutile, anatase), and as examples of scaly non-magnetic particles, graphite, mica, boron nitride, etc. are exemplified.
Even when acicular non-magnetic particles such as .alpha.-ferrite which are described in Japanese Patent Application Laid-Open (KOKAI) No. 4-325915 (1992) are used, it is difficult to obtain the desired dispersibility, so that the surface smoothness and the strength of such a magnetic medium having a non-magnetic undercoat layer are insufficient.
Use of coated .alpha.-Fe.sub.2 O.sub.3 is proposed in Japanese Patent Application Laid-Open (KOKAI) No. 62-295217 (1987). It discloses a magnetic recording medium comprising a substrate and a magnetic coating layer containing magnetic particles and a binder, wherein the magnetic coating layer contains .alpha.-Fe.sub.2 O.sub.3 particles which are coated with a compound containing at least one selected from the group consisting of Si, Cr and Al.
Use of coated .alpha.-Fe.sub.2 O.sub.3, however, not only impairs the improvement of the recording density in the same way as addition of non-magnetic carbon black, but also impairs the reduction in the thickness of the film. As described above, the magnetization thickness from the surface of the magnetic tape is reduced and the reduction in the thickness of the film is insufficient.
In addition, in Japanese Patent Application Laid-Open (KOKAI) No. 62-295217 (1987), there is no description of the formation of a non-magnetic undercoating layer on the non-magnetic substrate. Also, there is no description which motivates the addition of non-magnetic particles composed of .alpha.-Fe.sub.2 O.sub.3 particles which are coated with a compound containing at least one selected from the group consisting of Si, Cr and Al to a non-magnetic undercoating layer. In other words, there is no description which motivates a combination of Japanese Patent Application Laid-Open (KOKAI) Nos. 63-187418 (1988), 4-167225 (1992) and 4-325915 (1992). That is, Japanese Patent Application Laid-Open (KOKAI) No. 62-295217 (1987) describes the addition of coated .alpha.-Fe.sub.2 O.sub.3 particles to a magnetic coating layer only in order to provide the magnetic coating layer with an excellent wear resistance and an appropriate magnetic head-abrasive property.
Accordingly, non-magnetic particles which impart to an undercoating layer excellent properties such as an excellent surface smoothness and a high strength, a non-magnetic undercoating layer having an excellent surface smoothness and a high strength so as to provide such a magnetic recording medium and a magnetic recording medium having a small light transmittance, an excellent surface smoothness and a uniform thickness are now strongly demanded.
As a result of studies undertaken by the present inventors so as to achieve such demands, it has been found that by making non-magnetic particles obtained by coating acicular or spindle-shaped .alpha.-Fe.sub.2 O.sub.3 particles with at least one selected from the group consisting of an Al compound, a silane coupling agent, a titanate coupling agent, a aluminate coupling agent and a zirconate coupling agent, contain in a non-magnetic undercoating layer for a magnetic recording medium, it is possible to produce a magnetic recording medium which has a base film having a strength high enough as a base film for a magnetic recording medium and an excellent surface smoothness, and which has a small light transmittance, a high smoothness and a uniform thickness. On the basis of this finding, the present invention has been achieved.